I would stay with the Aqua C. The Urchin is an insump model and if it really a urchin and not urchin pro it should be prefectly sized. Aqua C rates it for 20-75 gallons. I havent ever heard anything bad about aqua C skimmers. I know I personally have used the larger Aqua C skimmers and havent ever had a problem but those were with the EV series. But if you feel more comfortable with Euro Reef that is ultimately what will be more important, but it wouldnt hurt to try out the Aqua C during the setup process of your tank, and potentially save you some money for coral instead of another skimmer.

As far as the mh goes I would stick with the 150 DE halide. I would just think you would run into too many problems (heat, light acclimation, etc) with the 250w.
The DI resin (the color changing stuff) should still be good so long as it was in a closed container where the humidity couldnt effect it.